DESCRIPTION (Verbatim from the Applicant's Abstract): Advances in ultrasound have increased the frequency of antenatal diagnosis of complete heart block with heart failure and hydrops fetalis Because no cardiac pacemaker modality exists for intrauterine application, these patients die prenatally or postnatally. A Monolithic Fetal Pacemaker System has been designed, capable of intrauterine placement, that avoids the problems incident to an exposed lead This is applicable to fetal heart block, where pacing for 2- 4 weeks prior to Cesarean delivery should resolve hydrops fetalis permitting survival. This system can also be used prophylactically for avoiding death after fetal surgery. The pacemaker is novel because it employs an intrapleural electrode to pace heart through the pericardium, because it uses an extraordinary pathway of current passage, traversing fetal skin, and because its low profile monolithic character, permits minimally invasive implantation while avoiding cord complications. This proposed research (1) develops techniques of ultrasound-guided placement of the prototype Monolithic Fetal Pacemaker, (2) creates prototypes, and (3) in a study in pregnant sheep, leads to development of surgical techniques of deployment. If successful, this research will lead to important advances in fetal medicine and surgery, permitting survival of fetuses with complete heart block and enhancing survival of fetuses that undergo fetal surgery. PROPOSED COMMERCIAL APPLICATION: The Monolithic Fetal Pacemaker will be used by fetal interventionalists and perinatologists in fetuses with complete heart blocck. It may become a mainstay of fetal surgery, where it will be prophylactically placed at the time of fetal surgery. Broad opportunities for commercialization exit, because of the wide range of its use and the international network of clinicians in the fields of Obstetrics, Cardiac Surgery, Pediatric Cardiology, Electrophysiology and Fetal Surgery.